Crawling is one of the most frustrating glaze defects — your glaze pulls back from the clay surface during firing, leaving bare patches and bead-like edges. Understanding why it happens makes it straightforward to prevent. Here are the main causes and how to fix each one.
What is Glaze Crawling?
Crawling occurs when molten glaze pulls away from the clay surface and beads up rather than spreading evenly. It leaves bare spots of unglazed clay surrounded by thick ridges of glaze. In severe cases it looks like the glaze has retreated to one area of the piece entirely.
Causes and Fixes
1. Glaze Applied Too Thick
The most common cause. A thick glaze layer shrinks significantly during drying and firing — if it shrinks more than the clay beneath it, the bond breaks and the glaze crawls. Apply glaze in thinner, even coats. Check your specific gravity — most dipping glazes work best between 1.40–1.50.
2. Dusty or Oily Bisqueware
Glaze does not bond well to a dusty, greasy, or oily bisque surface. Fingerprints from handling, dust from shelf grinding, or residue from kiln wash can all prevent proper adhesion. Wipe bisqueware with a clean damp sponge before glazing and avoid handling with bare hands after cleaning.
3. High Clay Content in the Glaze
Glazes with a high percentage of raw clay — particularly EPK or ball clay — shrink significantly during drying. If the glaze cracks while drying on the bisqueware, those cracks become the sites where crawling starts during firing. Try calcining part of the clay content in your glaze recipe — calcined clay has already been through the shrinkage process and reduces cracking on drying.
4. Glaze Applied Over an Unfired Glaze Layer
Layering two glazes can cause crawling if the top layer does not bond properly to the first. Allow the first glaze coat to dry completely before applying a second. Some glaze combinations are simply incompatible — test before committing to a full piece.
5. Firing Too Fast in the Early Stages
Firing quickly through the 212–500°F range can cause the glaze surface to set before gases from the clay fully escape, disrupting the glaze bond. Slow your initial ramp rate.
How to Fix a Crawled Piece
Minor crawling can sometimes be repaired. Let the piece cool completely, grind the crawled edges smooth with a diamond pad, apply fresh glaze to the bare areas, and refire. Results vary — severe crawling is often difficult to fully repair.
I am a visually impaired ceramic artist. I have been making for around 8 years now. I specialize in functional colorful pottery. Mainly nerikome and other decorative processes.
